Applicability of the Zintl Concept to Understanding the Crystal Chemistry of Lithium-Rich Germanides and Stannides.

With this contribution, we take a new, critical look at the structures of the binary phases Li5Ge2 and Li5Sn2. Both are isostructural (centrosymmetric space group R3̅m, no. 166), and in their structures, all germanium (tin) atoms are dimerized. Application of the valence rules will require the allocation of six additional valence electrons per [Ge2] or [Sn2] unit considering single covalent bonds, akin to those in the dihalogen molecules. Alternatively, four additional valence electrons per [Ge2] or [Sn2] anion will be needed if homoatomic double bonds exist, in an analogy with dioxygen. Therefore, five lithium atoms in one formula unit cannot provide the exact number of electrons, leaving open questions as to what is the nature of the chemical bonding within these moieties. Additionally, by means of single-crystal X-ray diffraction, synchrotron powder X-ray diffraction, and neutron powder diffraction, we established that the Li and Sn atoms in Li5Sn2 are partially disordered, i.e., the actual chemical formula of this compound is Li5-xSn2+x (0 < x < 0.1). The convoluted atomic bonding in the case where tin atoms partially displace lithium atoms results in the formation of larger covalently bonded fragments. Our first-principle calculations suggest that such disorder leads to electron doping. Contrary to that, both experimental and computational findings indicate that in the Li5Ge2 structure, the [Ge2] dimers are slightly oxidized, i.e., hole-doped, as a result of approximately 30% vacancies on a Li site, i.e., the actual chemical formula of this compound is Li5-xGe2 (x ≈ 0.3).

Advancing Heteroanionicity in Zintl Phases: Crystal Structures, Thermoelectric and Magnetic Properties of Two Quaternary Semiconducting Arsenide Oxides, Eu8Zn2As6O and Eu14Zn5As12O.

Two novel quaternary oxyarsenides, Eu8Zn2As6O and Eu14Zn5As12O, were synthesized through metal flux reactions, and their crystal structures were established by single-crystal X-ray diffraction methods. Eu8Zn2As6O crystallizes in the orthorhombic space group Pbca, featuring polyanionic ribbons composed of corner-shared triangular [ZnAs3] units, running along the [100] direction. The structure of Eu14Zn5As12O crystallizes in the monoclinic space group P2/m and its anionic substructure can be described as an infinite "ribbonlike" chain comprised of [ZnAs3] trigonal-planar units, although the structural complexity here is greater and also amplified by disorder on multiple crystallographic positions. In both structures, the O2- anion occupies an octahedral void with six neighboring Eu2+ cations. Formal electron counting, electronic structure calculations, and transport properties reveal the charge-balanced semiconducting nature of these heteroanionic Zintl phases. High-temperature thermoelectric transport properties measurements on Eu14Zn5As12O reveal relatively high resistivity (ρ500K = 8 Ω·cm) and Seebeck coefficient values (S500K = 220 µV K-1), along with a low concentration and mobility of holes as the dominant charge-carriers (n500K = 8.0 × 1017 cm-3, µ500K = 6.4 cm2/V s). Magnetic studies indicate the presence of divalent Eu2+ species in Eu14Zn5As12O and complex magnetic ordering, with two transitions observed at T1 = 21.6 K and T2 = 9 K.

Yet Another Case of Lithium Metal Atoms and Germanium Atoms Sharing Chemistry in the Solid State: Synthesis and Structural Characterization of Ba2 LiGe3.

Several Ba-Li-Ge ternary phases are known and structurally characterized, including the title compound Ba2 LiGe3 . Its structure is reported to contain [Ge6 ]10- anions that exhibit delocalized bonding with a Hückel-like aromatic character. The Ge atoms are in the same plane with the Li atoms, and if both types of atoms are considered as covalently bonded, [LiGe3 ]4- honeycomb-like layers will result. The latter are separated by slabs of Ba2+ cations. However, based on the systematic work detailed herein, it is necessary to re-evaluate the phase as Ba2 Li1-x Ge3+x (x<0.05). Although small, the homogeneity range is clearly demonstrated in the gradual change of the unit cell for four independent samples. Subsequent characterization by single-crystal X-ray diffraction methods shows that the Ba2 Li1-x Ge3+x structure, responds to the varied number of valence electrons and the changes are most pronounced for the refined lengths of the Li-Ge and Ge-Ge bonds. Indirectly, the changes in the Ge-Li/Ge distances within layers affect the stacking too, and these changes can be correlated to the variation of the c-cell parameter. Chemical bonding analysis based on TB-LMTO-ASA level calculations affirms the notion for covalent character of the Ge-Ge bonds; the Ba-Ge and Li-Ge interactions also show some degree of covalency.

Electrochemical Flux Synthesis of Type-I Na8Si46 Clathrates: Particle Size Control Using a Solid or Molten Na-Sn Mass Transport Mediator.

Sodium-filled silicon clathrates have a host of interesting properties for thermoelectric, photovoltaic, and battery applications. However, the metastability of the clathrates has made it difficult to synthesize them with the desired morphology and crystallite size. Herein, we demonstrate an electrochemical method whereby Na4Si4 dissolved in a Sn-based flux is converted to the Na8Si46 type-I clathrate using galvanostatic (constant current) oxidation. The temperature has a large influence on the products, with the reactions at 485 °C resulting in clathrates with small particle sizes (1-2 µm), while larger single crystals are obtained at 538 °C. The difference in microstructure is attributed to the solid vs liquid state of the Na-Sn phase at the reaction temperature, which is supported by the observed voltage profiles. The demonstrated method is promising for the tunable growth of Si clathrates and could be applicable to a broad range of intermetallic compounds.

Structural Complexity and Tuned Thermoelectric Properties of a Polymorph of the Zintl Phase Ca2CdSb2 with a Non-centrosymmetric Monoclinic Structure.

The Zintl phase Ca2CdSb2 was found to be dimorphic. Besides the orthorhombic Ca2CdSb2 (-o), here we report on the synthesis, the structural characterization, and the thermoelectric transport properties of its monoclinic form, Ca2CdSb2 (-m), and its Lu-doped variant Ca2-xLuxCdSb2 (x ≈ 0.02). The monoclinic structure exhibits complex structural characteristics and constitutes a new structure type with the non-centrosymmetric space group Cm (Z = 30). The electrical resistivity ρ(T) measured on single crystals of both phases portrays a transition from a semiconductor to a degenerate p-type semiconductor upon doping with Lu and with an attendant change in the Hall carrier concentration nH from 7.15 × 1018 to 2.30 × 1019 cm-3 at 300 K. The Seebeck coefficient S(T) of both phases are comparable and indicate a hole-dominated carrier transport mechanism with magnitudes of 133 and 116 µV/K at 600 K for Ca2CdSb2 (-m) and Ca2-xLuxCdSb2, respectively. The convoluted atomic bonding with an attendant large unit cell volume of ∼4365 Å3 drives a putative low thermal conductivity in these materials resulting in a power factor PF of 1.63 µW/cm K2 and an estimated thermoelectric figure of merit zT of ∼0.5 for Ca2-xLuxCdSb2 at 600 K. Differential scanning calorimetry results reveal the stability of these phases up to about 960 K, making them candidates for moderate temperature thermoelectric materials.

Synthesis of Type II Ge and Ge-Si Alloyed Clathrates Using Solid-State Electrochemical Oxidation of Zintl Phase Precursors.

Germanium clathrates with the type II structure are open-framework materials that show promise for various applications, but the difficulty of achieving phase-pure products via traditional synthesis routes has hindered their development. Herein, we demonstrate the synthesis of type II Ge clathrates in a two-electrode electrochemical cell using Na4Ge4-ySiy (y = 0, 1) Zintl phase precursors as the working electrode, Na metal as the counter/reference electrode, and Na-ion conducting ß″-alumina as the solid electrolyte. The galvanostatic oxidation of Na4Ge4 resulted in voltage plateaus around 0.34-0.40 V vs Na/Na+ with the formation of different products depending on the reaction temperature. When using Na4Ge3Si as a precursor, nearly phase-pure, alloyed type II Ge-Si clathrate was obtained at 350 °C. The Na atoms in the large (Ge,Si)28 cages of the clathrate occupied off-centered positions according to Rietveld refinement and density functional theory calculations. The results indicate that electrochemical oxidation of Zintl phase precursors is a promising pathway for synthesizing Ge clathrates with type II structure and that Si alloying of the Zintl phase precursor can promote selective clathrate product formation over other phases.

Giant Thermoelectric Power Factor Anisotropy in PtSb1.4Sn0.6.

We report on the giant anisotropy found in the thermoelectric power factor (S2σ) of marcasite structure-type PtSb1.4Sn0.6 single crystal. PtSb1.4Sn0.6, synthesized using an ambient pressure flux growth method upon mixing Sb and Sn on the same atomic site, is a new phase different from both PtSb2 and PtSn2, which crystallize in the cubic Pa3̅ pyrite and Fm3̅m fluorite unit cell symmetry, respectively. The large difference in S2σ for heat flow applied along different principal directions of the orthorhombic unit cell stems mostly from anisotropic Seebeck coefficients.

Caught in Action. The Late Rare Earths Thulium and Lutetium Substituting Aluminum Atoms in the Structure of Ca14AlBi11.

Described are two unprecedented cases, where the rare earth metals Tm and Lu partially substitute Al atoms in the structure of the Zintl phase Ca14AlBi11. These are the first examples within this large family where lanthanides replace the atoms of a main group element. Such crystal chemistry has never been observed before in other materials with the same structure. The uniqueness of this finding is also amplified by the fact that the rare earth metal atoms in the crystal structure are tetrahedrally coordinated, which is another remarkable trait of the new Tm- and Lu-substituted compounds.

Complex Structural Disorder in the Zintl Phases Yb10MnSb9 and Yb21Mn4Sb18.

A systematic investigation of the ternary system Yb-Mn-Sb led to the discovery of the novel phase Yb10MnSb9. Its crystal structure was characterized by single-crystal X-ray diffraction and found to be complex and highly disordered. The average Yb10MnSb9 structure can be considered to represent a defect modification of the Ca10LiMgSb9 type and to crystallize in the tetragonal P42/mnm space group (No. 136) with four formula units per cell. The structural disorder can be associated with both occupational and positional effects on several Yb and Mn sites. Similar traits were observed for the structure of the recently reported Yb21Mn4Sb18 phase (monoclinic space group C2/c, No. 15), which was reevaluated as part of this study as well. In both structures, distorted Sb6 octahedra centered by Yb atoms and Sb4 tetrahedra centered by Mn atoms form disordered fragments, which appear as the hallmark of the structural chemistry in this system. Discussion along the lines of how difficult, and important, it is to distinguish Yb10MnSb9 from the compositionally similar binary Yb11Sb10 and ternary Yb14MnSb11 compounds is also presented. Preliminary transport measurements for polycrystalline Yb10MnSb9 indicate high values of the Seebeck coefficient, approaching 210 µV K-1 at 600 K, and a semiconducting behavior with a room-temperature resistivity of 114 mΩ cm.

Two Polymorphs of BaZn2P2: Crystal Structures, Phase Transition, and Transport Properties.

The novel α-BaZn2P2 structural polymorph has been synthesized and structurally characterized for the first time. Its structure, elucidated from single crystal X-ray diffraction, indicates that the compound crystallizes in the orthorhombic α-BaCu2S2 structure type, with unit cell parameters a = 9.7567(14) Å, b = 4.1266(6) Å, and c = 10.6000(15) Å. With ß-BaZn2P2 being previously identified as belonging to the ThCr2Si2 family and with the precedent of structural phase transitions between the α-BaCu2S2 type and the ThCr2Si2 type, the potential for the pattern to be extended to the two different structural forms of BaZn2P2 was explored. Thermal analysis suggests that a first-order phase transition occurs at ∼1123 K, whereby the low-temperature orthorhombic α-phase transforms to a high-temperature tetragonal ß-BaZn2P2, the structure of which was also studied and confirmed by single-crystal X-ray diffraction. Preliminary transport properties and band structure calculations indicate that α-BaZn2P2 is a p-type, narrow-gap semiconductor with a direct bandgap of 0.5 eV, which is an order of magnitude lower than the calculated indirect bandgap for the ß-BaZn2P2 phase. The Seebeck coefficient, S(T), for the material increases steadily from the room temperature value of 119 µV/K to 184 µV/K at 600 K. The electrical resistivity (ρ) of α-BaZn2P2 is relatively high, on the order of 40 mΩ·cm, and the ρ(T) dependence shows gradual decrease upon heating. Such behavior is comparable to those of the typical semimetals or degenerate semiconductors.

Structural Uniqueness of the [Nb(As5)2]5- Cluster in the Zintl Phase Cs5NbAs10.

The structure of the novel Zintl phase, Cs5NbAs10, is reported for the first time. This compound crystallizes in the monoclinic P21/c space group (no. 14) with eight formula units per cell. The structure represents a unique atomic arrangement, constituting a new structure type with Wyckoff sequence e32. The most important structural element is the unprecedented [Nb(As5)2]5- cluster anion, formed by a Nb atom enclosed between two As5 rings. These nonaromatic cyclic species, formally [As5]5-, adopt an envelope conformation similar to that of cyclopentane. To date, it is only the second example of an [As5]5- ring with this conformation, reported in an inorganic solid-state compound. The bonding characteristics of the [Nb(As5)2]5- cluster and the [As5]5- rings are thoroughly investigated using first-principles methods and discussed. Electronic band structure calculations on Cs5NbAs10 suggest that this compound is a semiconductor with an estimated band gap of ca. 1.4 eV.

Bismuth as a Reactive Solvent in the Synthesis of Multicomponent Transition-Metal-Bearing Bismuthides.

Employment of liquid bismuth (Bi) allows the facile single-crystal growth of compounds containing elements with high melting points, provided that these elements have reasonably high solubility in Bi. Utilization of the Bi flux approach yielded two new ternary bismuthides, SrNi0.17(1)Bi2 [a defect variant of the BaCuSn2 type, space group Cmcm, a = 4.879(2) Å, b = 17.580(6) Å, and c = 4.696(2) Å] and CaTi3Bi4 [NdTi3(Sn0.1Sb0.9)4 structure type, space group Fmmm, a = 5.6295(7) Å, b = 9.8389(1) Å, and c = 23.905(3) Å]. In addition, the ternary antimonide CaV3Sb4, isostructural with CaTi3Bi4, was synthesized from antimony (Sb) flux, and analyzed with the goal of validating structural assessment of the bismuthide analogue, where the X-ray crystallographic work proved to be very challenging. All synthesized compounds exhibit complex crystal structures featuring quasi-two-dimensional building blocks of different topologies. First-principle calculations reveal hypervalent bonding in the homoatomic Bi subunits. Physical property measurements indicate metallic conductivity and the absence of localized magnetism in the studied compounds.

One Structure, Two Elements-LuGe2 Superconductor vs Ordinary Metallic Conductor LuSn2. A Case Study on How Site-Selective Germanium for Tin Atom Substitution Leads to Modulating of the Charge Distribution.

The substitution of chemically similar elements in a given crystal structure is an effective way to enhance physical properties, but the understanding on such improvements is usually impeded because the substitutions are random, and the roles of the different atoms cannot be distinguished by crystallographic symmetry. Herein, we provide a detailed crystallographic analysis and property measurements for the continuous solid solutions LuGexSn2-x (0 < x < 2). The results show that there is no apparent change of the global symmetry, with the end-members LuGe2 and LuSn2, as well as the intermediate LuGexSn2-x compositions adopting the ZrSi2 type structure (space group Cmcm, Pearson index oC12). Yet, the refinements of the crystal structures from single-crystal X-ray diffraction data show that Ge-Sn atom substitutions are not random, but occur preferentially at the zigzag chain. The patterned distribution of two group 14 elements leads to a significant variation in chemical bonding and charge ordering within the other structural fragment, the 2D square nets, thereby resulting in tuned electron transport. The enhancement is greater than that for the typical Bloch-Gruneisen model and more akin to that for the parallel-resistor model. Magnetization measurements on single crystals show bulk superconductivity in all LuGexSn2-x samples with shielding fractions as high as 90%. Specific heat data confirm the effect to originate from residual metallic tin in the material, indicating that Sn atom substitutions in the 2D square nets cause disruptions of the hypervalent bonding and local anisotropy, which ultimately leads to vanishing of the superconducting state in the end-member LuGe2. This work sheds light on how the complexity in chemical interactions by two different carbon congeners leads to changes in the physical properties and how they can be correlated with the induced charge distribution. These studies also provide a general approach to modulation of charge density and. thus, of emerging physical properties in other classes of intermetallic systems based on the main-group elements of groups 13 to 15.

From the Ternary Phase Ca14Zn1+δSb11 (δ ≈ 0.4) to the Quaternary Solid Solutions Ca14- xRE xZnSb11 (RE = La-Nd, Sm, Gd, x ≈ 0.9). A Tale of Electron Doping via Rare-Earth Metal Substitutions and the Concomitant Structural Transformations.

The ternary compound Ca14Zn1.37(1)Sb11 and its six rare-earth metal substituted derivatives Ca14- xRE xZnSb11 (RE = La-Nd, Sm, Gd; x ≈ 0.90 ± 0.06) have been synthesized and structurally characterized by single-crystal X-ray diffraction methods. All compounds formally crystallize in the tetragonal Ca14AlSb11 structure type (space group I41/ acd, No. 142, Z = 8). The crystal structure of Ca14Zn1.37(1)Sb11 subtly differs from the structure of the remaining six, as well as from the structure of the archetype, due to the presence of a partially occupied interstitial Zn position. The extra zinc atom is needed in this structure to alleviate the unfavorable number of valence electrons in the imaginary Ca14ZnSb11. Electron doping, via substitution of RE3+ ions on Ca2+ sites, is shown as an alternative route to achieve electron balance in these Zn-based analogs of the Ca14AlSb11 structure, which does not require the incorporation of interstitial atoms. Electrical resistivity measurements done on single-crystalline samples are in agreement with the notion that Ca14- xRE xZnSb11 moieties behave as either bad metals or heavily doped semiconductors. Magnetization measurements show Curie-Weiss paramagnetic behavior related to the local-moment magnetism of the RE3+ ions.

Layered Quaternary Germanides-Synthesis and Crystal and Electronic Structures of AELi2In2Ge2 ( AE = Sr, Ba, Eu).

Three new quaternary germanides with the composition AELi2In2Ge2 ( AE = Sr, Ba, Eu) have been synthesized and structurally characterized. The layered crystal structure of these phases features homoatomic In-In bonding, but there are no direct Ge-Ge bonds. Such a crystallographic arrangement can be regarded as an ordered quaternary derivative of the CaCu4P2 structure (trigonal syngony, Pearson code hR7). Comprehensive analysis of the structural genealogy suggests relationships with the structures of other layered pnictides and chalcogenides, which are discussed. Partitioning of the available valence electrons and the assignment of the formal charges indicate that the composition of the new germanides is charge-balanced. First-principles calculations and electrical transport measurements indicate poor metallic behavior, resulting from significant hybridization of the electronic states.

Synthesis, and Crystal and Electronic Structures, of the Titanium-Rich Bismuthides AE3Ti8Bi10 ( AE = Sr, Ba, Eu).

Three isotypic compounds with the chemical formula AE3Ti8Bi10 ( AE = Sr, Ba, Eu) have been obtained via both high-temperature solid state and flux growth reactions. Their crystal structure, representing a new type (space group P63/ mmc, Pearson symbol hP42), features an open framework composed of interlinked TiBi5 square pyramids and TiBi6 octahedra. The Ti-Bi substructure is penetrated by infinite columns of face-sharing AE6 polyhedra centered by Bi atoms. First-principle calculations and physical property measurements indicate metallic behavior and absence of localized magnetic moments on the Ti atoms. Analysis of the chemical bonding reveals strong Ti-Bi and Ti-Ti bonds. The latter demonstrate classic two-center, as well as multicenter, interactions.

Intricate Li-Sn Disorder in Rare-Earth Metal-Lithium Stannides. Crystal Chemistry of RE3Li4- xSn4+ x (RE = La-Nd, Sm; x < 0.3) and Eu7Li8- xSn10+ x ( x ≈ 2.0).

Reported are the syntheses, crystal structures, and electronic structures of six rare-earth metal-lithium stannides with the general formulas RE3Li4- xSn4+ x (RE = La-Nd, Sm) and Eu7Li8- xSn10+ x. These new ternary compounds have been synthesized by high-temperature reactions of the corresponding elements. Their crystal structures have been established using single-crystal X-ray diffraction methods. The RE3Li4- xSn4+ x phases crystallize in the orthorhombic body-centered space group Immm (No. 71) with the Zr3Cu4Si4 structure type (Pearson code oI22), and the Eu7Li8- xSn10+ x phase crystallizes in the orthorhombic base-centered space group Cmmm (No. 65) with the Ce7Li8Ge10 structure type (Pearson code oC50). Both structures can be consdered as part of the [RESn2] n[RELi2Sn] m homologous series, wherein the structures are intergrowths of imaginary RESn2 (AlB2-like structure type) and RELi2Sn (MgAl2Cu-like structure type) fragments. Close examination the structures indicates complex occupational Li-Sn disorder, apparently governed by the drive of the structure to achieve an optimal number of valence electrons. This conclusion based on experimental results is supported by detailed electronic structure calculations, carried out using the tight-binding linear muffin-tin orbital method.

An Unusual Triple-Decker Variant of the Tetragonal BaAl4-Structure Type: Synthesis, Structural Characterization, and Chemical Bonding of Sr3Cd8Ge4 and Eu3Cd8Ge4.

Reported are the synthesis and the crystal structures of the new ternary phases Sr3Cd8Ge4 and Eu3Cd8Ge4. The structures of both compounds have been established by single-crystal and powder X-ray diffraction methods. They crystallize in the tetragonal space group I4/mmm (No. 139, own structure type, Pearson symbol tI30) with Z = 2, and lattice parameters as follows: a = 4.4941(14) Å; c = 35.577(7) Å for Sr3Cd8Ge4, and a = 4.4643(12) Å; c = 35.537(9) Å for Eu3Cd8Ge4, respectively. The most prominent feature of the structure is the complex [Cd2Ge] polyanionic framework, derived by unique ordering of the Cd and Ge atoms in fragments that bear resemblance to the BaAl4 structure type. Temperature dependent DC magnetization measurements indicate that Eu3Cd8Ge4 displays Curie-Weiss paramagnetic behavior with no sign of magnetic ordering in the measured range. Theoretical considerations of the electronic structure on the basis of the tight-binding linear muffin-tin orbital (TB-LMTO-ASA) method are also presented and discussed.

The Ternary Alkaline-Earth Metal Manganese Bismuthides Sr2MnBi2 and Ba2Mn1-xBi2 (x ≈ 0.15).

Two new ternary manganese bismuthides have been synthesized and their structures established based on single-crystal X-ray diffraction methods. Sr2MnBi2 crystallizes in the orthorhombic space group Pnma (a = 16.200(9) Å, b = 14.767(8) Å, c = 8.438(5) Å, V = 2018(2) Å3; Z = 12; Pearson index oP60) and is isostructural to the antimonide Sr2MnSb2. The crystal structure contains corrugated layers of corner- and edge-shared [MnBi4] tetrahedra and Sr atoms enclosed between these layers. Electronic structure calculations suggest that Sr2MnBi2 is a magnetic semiconductor possessing Mn2+ (high-spin d5) ions, and its structure can be rationalized within the Zintl concept as [Sr2+]2[Mn2+][Bi3-]2. The temperature dependence of the resistivity shows behavior consistent with a degenerate semiconductor/poor metal, and magnetic susceptibility measurements reveal a high degree of frustration resulting from the two-dimensional nature of the structure. The compositionally similar Ba2Mn1-xBi2 (x ≈ 0.15) crystallizes in a very different structure (space group Imma, a = 25.597(8) Å, b = 25.667(4) Å, c = 17.128(3) Å, V = 11253(4) Å3; Z = 64; Pearson index oI316) with its own structure type. The complex structure boasts Mn atoms in a variety of coordination environments and can be viewed as consisting of two interpenetrating 3D frameworks, linked by Bi-Bi bonds. Ba2Mn1-xBi2 can be regarded as a highly reduced compound with anticipated metallic behavior.

Quaternary pnictides with complex, noncentrosymmetric structures. Synthesis and structural characterization of the new Zintl phases Na11Ca2Al3Sb8, Na4CaGaSb3, and Na15Ca3In5Sb12.

Three new Zintl phases, Na11Ca2Al3Sb8, Na4CaGaSb3, and Na15Ca3In5Sb12, have been synthesized by solid-state reactions, and their structures have been determined by single-crystal X-ray diffraction. Na11Ca2Al3Sb8 crystallizes with its own structure type (Pearson index oP48) with the primitive orthorhombic space group Pmn2(1) (No. 31). The structure is best viewed as [Al3Sb8](15-) units of fused AlSb4 tetrahedra, a novel type of Zintl ion, with Na(+) and Ca(2+) cations that solvate them. Na4CaGaSb3 also crystallizes in its own type with the primitive monoclinic space group Pc (No. 7; Pearson index mP36), and its structure boasts one-dimensional [GaSb3](6-) helical chains of corner-shared GaSb4 tetrahedra. The third new compound, Na15Ca3In5Sb12, crystallizes with the recently reported K2BaCdSb2 structure type (space group Pmc2(1); Pearson index oP12). The Na15Ca3In5Sb12 structure is based on polyanionic layers made of corner-shared InSb4 tetrahedra. Approximately one-sixth of the In sites are vacant in a statistical manner. All three structures exhibit similarities to the TiNiSi structure type, and the corresponding relationships are discussed. Electronic band structure calculations performed using the tight-binding linear muffin-tin orbital atomic sphere approximation method show small band gaps for all three compounds, which suggests intrinsic semiconducting behavior for these materials.